Conventionally, aluminum members are joined by brazing.
With the method of joining the aluminum members by brazing, since it is necessary to melt a brazing filler metal to thereby join the aluminum members (at a temperature of 580° C. to 620° C.), it will cause a positional accuracy problem, and the range of application of such method is limited. Further, in the method of joining the aluminum members by brazing, since a flux containing fluoride is used, it will cause a corrosion problem due to flux residue, not to mention that the flux is harmful to the human body; therefore, there is a desire to abolish such a method. Further, with the method of joining the aluminum members by brazing, since an intermetallic compound (which is a material having brittle property) is generated between the brazing filler metal and aluminum matrix, it will cause a problem that the joining portion will become brittle.
There are also other methods for joining metal members, such as laser welding, friction stir welding, and spot welding.
In these joining methods by welding, the area near the welding portion will become soft due to welding heat, and micro fissure will occur. Further, it will cause a corrosion problem due to clearance of the joining portion.
Further, due to high thermal conductivity of aluminum, large amount of power (i.e., heat input) is necessary.
As one of other methods for joining metal members, there is known a solid-phase diffusion joining method.
The solid-phase diffusion joining method is a method of heating and pressurizing the metal members to thereby join the metal members without melting the matrix and without causing noticeable deformation in solid-phase state. The feature of the solid-phase diffusion joining method is that the heat damage to the metal members can be reduced, wet spreading can be suppressed because the matrix is not molted, and precision assembly joining is possible.
However, in the case where the metal members are made of a metal likely to be oxidized, if the metal members are exposed to the atmospheric air, a hard natural oxide film (which is a joining inhibiting factor) will be formed on the surface of the metal members. In order to obtain a strong joining portion, it is necessary to increase joining pressure and joining temperature to mechanically break the oxide film, and that will inevitably increase deformation amount caused when joining the metal members.
To solve such problem, it is proposed that the joining surfaces of copper is treated with an oxide film removing liquid formed of an organic acid before performing solid-phase joining (see PTL 1, for example).
Further, it is proposed to pressurize to join the metal members with a sheet-like member sandwiched between the joining surfaces of the metal members, wherein the sheet-like member is made of a material (such as copper, zinc, silver, a copper alloy, a zinc alloy, a silver alloy, or silicon) that generates an eutectic reaction with aluminum (see PTL 2 and PTL 3, for example).